Posted by Jacob/CT on January 29, 2003 at 09:19:56:
In Reply to: Re: Heathkit/Vantage Pro comparisons posted by Richard Campbell In Vallejo, Calif. on January 28, 2003 at 21:29:55:
Interestingly enough, the source to which we seem to compare our readings to, the NWS ASOS stations, uses a 5 second average wind speed as its "instantaneous" reading according to section 3.2.2 (page 15) of the ASOS User's Guide.
Really the only type of anemometer that is closest to measuring instantaneous wind speed is the type that uses a motor and outputs electricity proportional (or some other known relationship after corrections are made) to the wind speed. All other types generally requires some kind of discrete event (i.e. a reed switch being closed or a photochopper beam being broken) in order to measure wind speed. This means that a sampling period of some sort is required since enough measurement time needs to elapse in order to have some data (otherwise you'll be left with unsmoothed data that really appears unnatural caused by the design of the measurement instrument). The more signals or events that are generated during a sampling period, the better the resolution of the instrument, and the smaller the sampling period the closer to "instantaneous" the measurement. Then it becomes a matter of what you define as a gust as there seems to be some discrepancy between official measurement method and perception.
I also think Richard's statements are true in that even an anemometer that could measure a gust that happened across 1/30th of a second it still wouldn't capture it accurately because there is going to be some "ramping up" time where the cups need to get spinning up to the full speed of the wind. I believe this in itself will make a spinning cup design inaccurate for gusts that last much less than a second. There is a different kind of anemometer that has 3 prongs that measures either the vibration of the air moving past or the amount of heat transferred...I really can't remember how it worked, but I would guess an instrument like that would do the trick and be able to measure fast and large fluctuations in wind speed since there are no moving parts.
: : "The 2-1/2 second update interval on the VP should be fast enough to capture high wind gusts. The inertial resistance of the vanes would make any faster update interval difficult to achieve."
: : I would have to disagree.
: : Wind velocity does change very quickly in gusty/squally conditions: just listen to it change pitch in your eardrum as it swirls around on a windy day! ( avery fast flutterling...and you can feel it shoving and pushing your around in sudden slams etc - i.e fro windspeeds 50 knots or higher). The faster you can sample it the better. I have a system that samples windspeed 30 times a second, and it picks up instantenous gusts up to 10 knots higher than a average over the whole second....by the tinmer between pusles being measured (not a davis sytem)....(but with a constant blowing from a reversed vacuum cleaner the 2 sample methods were the similar). So if you dont want to miss peak wind gusts, go with a heathkit or a Peet Bros weather station.
: : Just my thoughts!
: Hi Anon,
: Wow! A sampling rate of 30 times per second. Just from casual observation, I don't think the average weathervane even makes one revolution in 1/30 of a second under normal wind conditions. It's been too many years since I took first semester physics, but perhaps a recent graduate could calculate how much time it would take to change the angular momentum of a weather vane given its known mass. Inertial drag would limit the acceleration of the vane (One of Newton's immutable laws). I would think it would be impossible for a vane to increase its rotation speed fast enough to measure a 30 mph change in wind speed in 1 second, much less 1/30 of a second. Correspondingly, the moving air that the anemometer is attempting to measure has a cetain momemtum (mass and velocity) that must be conserved, so wind speed cannot instantly change.
: Richard
:
: : : : Has anyone seen the comparison of the Heathkit and Vantage Pro stations at this site for Taunton, Mass. http://www.thunderheadtech.com/Taunton2W/
: : : : Everything looks pretty close except for the peak wind gusts for the month. Heathkit had a max of 50 mph and V.P. had only 40 mph with higher gusts throughout. I’ll bet that the Heathkit has an instantaneous gust registering sample instead of Davis’ 2.25 second.
: : : : Does anyone have info on the Heathkit ID-5001 as to accuracy and specifications?
: : : My last weather station was a Heathkit. I threw away the specifications when I finally had to retire the station. The Zenith anemometer used a photo diode type tachometer that responded very quickly, at least until the vane bearings finally gave out. The Heathkit unit had either a gust or average mode similar to the Vantage Pro. I think that the Vantage Pro is more accurate than the Heathkit. I have used a mechanical air velocity gauge on both units. I checked The VP for accuracy with a fairly constant 25 mph wind and the station agreed with the mechanical measurement. The VP is much better than the Heathkit at picking up low wind speeds. The Heathkit had trouble picking up speeds less than 4mph.
: : : The 2-1/2 second update interval on the VP should be fast enough to capture high wind gusts. The inertial resistance of the vanes would make any faster update interval difficult to achieve.
: : : I suspect that the difference in wind speed that you are observing is a reflection of the actual values. Wind speed can vary greatly between two points a short distance apart. In particular, different elevations can make a substantial impact on wind speed, as can geographical obstacles such as buildings, trees, or hills.
: : : Richard
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